1. Field of the Invention
The present invention relates to a method for producing a copper indium selenium (CIS) or copper indium gallium selenium (CIGS) thin-film light-absorbing layer that can be applied to a thin-film solar cell, and a method for manufacturing a thin-film solar cell including the same.
2. Description of the Related Art
Solar cells are the devices that directly converts light energy into electrical energy. Solar cells have received the most intensive attention as potential systems for future energy production due to their ability to safely produce clean energy from the vast sunlight. Various kinds of inorganic and organic semiconductors have been applied to manufacture efficient solar cells. However, only a few solar cells using inorganic and organic semiconductors have been commercially successful to date, and typical examples thereof are silicon (Si) solar cells, copper indium selenium (CIS) solar cells and copper indium gallium selenium (CIGS) solar cells.
Silicon solar cells exhibit high photoelectric conversion efficiency but have the disadvantage of high manufacturing costs. Therefore, great effort to develop thin-film solar cells using compound semiconductors which are comparable with silicon solar cells has been made.
Among the various types of thin film solar cells under developing, typical examples are the solar cells comprised of a light-absorbing layer of Cu—In—Se (CIS) and Cu—In—Ga—Se (CIGS) systems, known as a CIS or CIGS thin film solar cells containing element groups of IB, IIIA, and VIA.
The CIS or CIGS thin-film solar cells are comprised of a thin-film light-absorbing layer of CuInxGa1-xSe2 (0≦x≦1), a thin-film buffer layer of CdS, and another layer of n-type compound semiconductor. Among them, a thin-film light-absorbing layer is most important because its characteristics determine the performance of CIS or CIGS solar cells. (Hereafter the thin-film solar cells or light-absorbing layers of CIS or CIGS systems are expressed simply as those of CIGS for convenience.)
A CIGS phase of thin-film light-absorbing layer has a chalcopyrite crystal structure whose band gap energy is in the range of 1.05 to 1.40 eV. Particularly, since the band gap energy of CIGS phase may be controlled by varying its chemical compositions, the photoelectric conversion of CIGS solar cells can be improved. In addition, a CIGS light-absorbing layer has high resistance under irradiation of electromagnetic waves such as X-ray and absorbs light efficiently, for example, more than 90% of incident light even with a thickness of 1 to 2 μm.
Manufacturing processes of CIGS solar cells can be classified into two groups of a vacuum process and a non-vacuum process.
In the vacuum process, elemental components of CIS or CIGS are evaporated under vacuum or at a low pressure (lower than 1 atm) and deposited on the surface of a substrate to form a CIGS light-absorbing layer (or film). Compared with non-vacuum process, the low-pressure process is much advantageous to obtain CIS or CIGS compounds with predetermined compositions in a single step. Additionally, target compositions of CIGS compounds may also be achieved through subsequent post-processing such as selenization. During vacuum processing, the elemental components can be supplied by various methods such as thermal vacuum evaporation, electron beam coating, sputtering, chemical vapor deposition (CVD), and metal-organic chemical vapor deposition (MOCVD). The band gap can be controlled in the thickness direction by the choice of processing methods. Therefore an adequate deposition processes and post-treatment can be chosen to obtain a desirable band gap profile of CIGS light-absorbing layer along the direction of thickness.
Many techniques related to low pressure processes have been reported. For example, Korean Patent No. 0933890 discloses a method for fabricating a CIGS thin film comprised of depositing copper indium (CuIn), copper gallium (CuGa) and a selenide compound on a substrate to form a precursor coat and subsequently heat-treating the precursor coat in a selenium atmosphere. Further, Korean Unexamined Patent Publication No. 2011-0055830 discloses a method for producing a thin-film light-absorbing layer of CIGS solar cells comprised of filling a CIGS powder into the evaporation pot in a furnace chamber; and directly evaporating the CIGS powder without composition change; and depositing the evaporated CIGS vapor onto the substrate to form a CIGS thin film.
Fabrication of CIGS light-absorbing layer by the low pressure or vacuum processes is beneficial to directly obtain a dense structure as well as to control composition in a continuous manner along the direction of thickness during deposition. On the other hand, it has drawbacks as follows: long processing time, high materials loss up to 20˜40%, and very complicated process control due to multicomponent systems of three or four components.
A non-vacuum process is suitable for the production of a CIGS light-absorbing layer at atmospheric pressure condition. A powder process, a typical example of non-vacuum processes, adopts a sintering method as a key step to form the CIGS phase from the powder mixtures of oxides (e.g., CuO, InO2, Ga2O3 and SeO2) or binary compounds (e.g., CuIn, CuGa, CuSe, InGa, InSe and GaSe) or metallic elements (e.g., Cu, In, Ga, Se) as precursor materials. In the powder process, the CIGS phase is formed through reduction of oxide precursors into metallic elements or subsequent inter-element or inter-phase diffusion between the binary compound precursors during heat-treatment at high temperature simultaneously with controlled atmosphere. Prior to sintering at high temperature to fabricate a high-density light-absorbing layer, it is necessary to make a high-density powder coat on the substrate. Various powder processes can be used to form dense layers from the powder precursors, depending on both material types and particle characteristics. Typical examples are as follows: screen printing or doctor blade methods using slurries of precursor powders with a suitable liquid medium including some kinds of organic binders in order to control its viscosity and binding capacity, an electrostatic spray method for spraying electrically charged particles over the surface of a substrate, and an electrophoresis method for depositing electrically charged particles on the surface of a substrate with help of applied electric field. Besides of these methods, organometallic precursors can be coated over the substrate and then thermally dissociated to form a CIGS light-absorbing layer.
Korean Patent No. 0989077 discloses a method for producing a CIGS thin film by paste coating method, instead of conventional vacuum deposition. The use of a CIS or CIGS thin film produced by the method reduces loss of the raw materials in the manufacture of solar cells and enables manufacture of large-area solar cells on a commercial scale.
A dense CIGS light-absorbing layer free of open pores penetrating through the film is necessary to obtain high efficiency CIGS solar cells. However, it is not easy to produce a dense CIGS light-absorbing layer by a powder method from the powder precursors or a paste or an ink including the powder precursors. In order to obtain a dense film without open pores penetrating the film by a powder process, either thick powder coats or sintering at high temperature are inevitably chosen, which causes detrimental effects of high material loss or thermal damage of the substrate.